1. Field of the Invention
The present invention relates to abnormality detecting devices capable of automatically detecting an occurrence of an abnormal state such as invasion of a person into a monitored area based on a video signal in the monitored area, such as a monitoring camera.
2. Description of Related Art
Conventionally, known are a variety of abnormality detecting devices which detect a presence of a movement of a body in a monitored area based on a video signal in the monitored area and detect an occurrence of an abnormal state based on the detecting result (for example, see Japanese Laid-Open Patent Publication No. 2000-115761).
As this kind of abnormality detecting device, for example, known is a monitoring camera which detects a presence of a movement of a body based on an amount of temporal alteration of brightness level of the video signal in the monitored area. In this monitoring camera, one image is divided into a plurality of blocks, and the amount of temporal alteration of the brightness level of the video signal is detected in every block. And when the amount of temporal alteration of the brightness level of the video signal exceeds a threshold, it is determined that there has been a movement of a body, resulting in generation of alarm sound. People around the area are thereby informed of the occurrence of an abnormal state.
However, in the conventional monitoring camera which detects a presence of a movement of a body based on the amount of temporal alteration of the brightness level of the video signal, in the case where it is very bright or very dark in the monitored area, even when an abnormal state occurs, the amount of temporal alteration of the brightness level of the video signal is small, and therefore, the occurrence of an abnormal state may not be detected. For example, in the case where a person wearing black clothes invades the monitored area where it is very dark, the amount of temporal alteration of the brightness level of the video signal does not exceed the threshold, and the occurrence of an abnormal state may not be detected.
On the other hand, in the case where the monitored area is in normal brightness, even when any abnormal state has not been occurred, the occurrence of an abnormal state is likely to be erroneously detected due to a large amount of temporal alteration of the brightness level of the video signal. For example, in the case where the strength of sunlight coming into the monitored area changes, the amount of temporal alteration of the brightness level of the video signal exceeds the threshold and the occurrence of an abnormal state may be erroneously detected.
Therefore, adjustment of the threshold used in the determination of a movement has been performed depending on the brightness in the monitored area. However, the above mentioned problems are still occurring.